


Check Sync

by SoloShikigami



Series: Heat Sync [3]
Category: Undertale (Video Game)
Genre: Alternate Universe - Underswap, CEO, M/M, Mating Cycles/In Heat, Mildly Dubious Consent, Underswap Papyrus, Underswap Sans
Language: English
Status: Completed
Published: 2016-07-05
Updated: 2016-07-05
Packaged: 2018-07-21 15:11:28
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 4,290
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/7392517
Author URL: https://archiveofourown.org/users/SoloShikigami/pseuds/SoloShikigami
Summary: <blockquote class="userstuff">
              <p>What happens if a monster doesn't take care of their heat? The increased magic builds up in their soul and it bursts, which could cause the monster serious harm or in the case of those with a low HP, death.<br/>Good thing Papyrus knows how and when to do a check.</p>
            </blockquote>





	Check Sync

**Author's Note:**

  * For [Eiznel24](https://archiveofourown.org/users/Eiznel24/gifts).



> The Heat Sync series isn’t related to Swap Synergy – the Swap Skellies haven’t screwed each other before this XD

 

The clock said that it was past three in the morning, but Sans had all but ignored it. He had sent his assistant home hours ago, insisting that he get some rest because the next day was going to be a long one.

Sans never minded the long hours; two to four hours of sleep and he was right as rain. In the morning, there was going to be an important meeting to update some of the investors as well as some of President Gaster’s representatives on the progress of their massive collaboration project. He was currently staring through the slide show that was going to aid him in giving his update and he knew that it was as pristine and perfect as anyone would be able to get it.

The CEO of S.W.A.P. Corp. was a prideful being. He always worked his hardest and strived to always be the best (he usually was), and he knew that his presentation for tomorrow would be his greatest to date – there was no need to go over it anymore. Despite that, he tried to focus intently on the images, going through his own speech over and over in a vain attempt to ignore something far, far more pressing.

It had been four days since he and the CEO of Edge Inc. had their own… private meeting, and it had soothed his heat long enough to think that it had dispersed and he happily thought he could continue on with business like always.

But it only satisfied Sans until that same evening. Since then he had been ignoring the waves that started out as uncomfortable extra warmth until his bones felt like they were on fire – the heat had even reached a point so intense that his heat scent burned away. He had been able to plaster his usual grin on and hide behind his work, even his assistant seemed to be none the wiser, and Sans intended to keep it that way.

Sans sighed, pushing his keyboard away with a little more force than was necessary to burying his face in his palms with a groan as another wave of fire threatened to overtake him.

“Oh, stars, would this please just stop?” he muttered. He finally glanced over at the clock as one hand slipped down to his tie and he started to loosen it. “I should probably try to get some sleep.”

The tie slipped off and was left carelessly on the edge of the desk. Sans kicked off his shoes, nudging them against the side of the plush, green couch he had in his office and collapsed onto the cushions. He spent a few minutes tossing and turning, trying to get comfortable, but it wasn’t working. With a grunt of frustration, he undid a few buttons on his shirt and yanked it over his head, leaving him in a white, sleeveless undershirt. He took off his belt as well but he refused to take off his pants; he didn’t care that it was his office and likely the only person who would see him in his underwear was Papyrus. Sans did have standards, after all.

But the heat still didn’t die down. He considered lowering the temperature in his office, again, but he was sure it didn’t go below sixty degrees.

With a pained whimper, Sans undid his pants and shoved his hands underneath to touch himself.

“Please, please work,” he begged, coaxing himself to get his magic to form what he felt he needed.

It took longer than he almost had patience for, but soon his hand was pulling his cyan-colored cock free from the confined of his boxers and pants.

The only sounds in the room for the next few moments were the fans from the central air units pumping cold air into the room and the desperate, panting moans from Sans.

With a strangled cry, he spilled his seed over his hands. He was still for a moment, catching his breath, and waiting for the fire to subside.

“Maybe just another minute,” he murmured, reaching down to grab his shirt off the floor to wipe off his hands and the edge of his boxers.

But the fire was only fanned.

Sans rolled over and shoved his face into the back of the couch with another groan of frustration. Thankfully, masturbating had made him just tired enough so that he could drop off into a fitful, lust-filled dreamscape.

*~*~*~*~*~*~*~*~*~*

As usual, Papyrus was barely paying any attention, unless one counted him endlessly staring at Sans as if he were a puzzle to solve. A very difficult puzzle.

Something was incredibly off with Sans. When Papyrus came in to drop off his usual morning coffee, he noticed that while Sans was dressed, there was a shirt scrunched up on the floor by the couch and one of his ties looked like it was tossed onto the edge of the desk.

When Papyrus asked, Sans just grinned and said that he hadn’t slept all night and was in a rush to get dressed before anyone showed up, but by golly was he ready for that meeting!

Papyrus decided to skip the question about why the room was ridiculously cold, especially since he was shooed out of the office to go make sure the meeting room was adequately prepared and to tuck in his shirt for stars sake!

He wouldn’t see Sans again until the meeting began. Sans had arrived last, shaking hands with all of those who came to the meeting. Papyrus was taken aback by a newcomer – another skeleton, dressed in all black with a white tie.

Apparently President Gaster had a son.

Sans immediately delved into his presentation and Papyrus was watching him closely. Few would probably notice the CEO with boundless energy was speaking a little quicker than normal (though one of the reps sitting next to Gaster’s son whispered that Sans must have had an extra cup of coffee that morning), it was doubtful that anyone would notice the very thing sheen of sweat building up on his forehead (it was summer and the humidity had been oppressive all week), but there was no possible way that anyone would know what Papyrus intended on learning.

It was one of those monster abilities that lay dormant in many of them; the ability to “check” someone. It was an old practice that was necessary in the days when monsters were proud warriors. Papyrus had learned about it as a teenager and taught himself how to do it as a form of fun and messing with unsuspecting humans. Some monsters caught on to when they were being checked.

Papyrus wondered if Sans would.

He leaned back in his chair and focused more on Sans, more specifically his soul. It took a little time, but when he did finally get what he wanted, it took everything in his body to not react to the ice cold spear that he felt impale him.

It was then that he noticed Sans had stopped talking. His eyes wavered and he looked a little ill.

“Sir?” one of their office workers who also sat in the meeting prompted.

Sans shook his head. “Sorry. What was I saying?”

The CEO turned to his presentation on the screen to try to re-orient himself, but the screen blurred.

Papyrus saw Sans shudder and he could tell he was trying his damndest to repress it. Sans pressed a hand to his temple.

“Sir?” the office worker prompted again, getting to their feet. Papyrus did too and starting moving towards his boss.

Sans waved the worker off. “I’m okay. I, uhm, I just thought of something important that has to be done. Most of the key items have been covered, you can wrap it up,” he turned to everyone with a grin, but those who knew better knew it was fake. “Please forgive me and excuse me. If you have any other questions, please speak to my assistant, Papyrus, after the presentation and he will be more than happy to schedule a phone call or meeting with me later today or tomorrow.”

The worker looked a little nervous, but nodded and began to carry on.

“Boss,” Papyrus whispered meeting Sans at the door.

“Stay,” Sans insisted. “I’m fine. You need to do what I said.”

“But-”

“Do what I said.”

Papyrus was stunned – he never heard that dark tone from Sans before.

Sans’ face softened again and he smiled before patting Papyrus on the shoulder. “I’m counting on you, Papy.”

Without another word, Sans was gone.

*~*~*~*~*~*~*~*~*~*

Sans was all smiles and quick steps as he made his way back to his office. As soon as the door shut, he dropped to his knees and wrapped his arms around himself.

“Oh, stars, what am I going to do?” he moaned.

Sans’ whole body shook, the heat was getting worse as he felt frozen and burning at the same time. He tried to get to his feet but he stumbled back to his knees. He tried crawling to the couch but he only made it to the middle of his floor before he had no choice but to collapse on his side, curling into a ball with a pained groan.

*~*~*~*~*~*~*~*~*~*

Papyrus spent the rest of the meeting feeling torn. If what he saw when he checked Sans was true, then he shouldn’t be stuck in this meeting room. There was a possibility that Papyrus was wrong, though. Also, if it was true, Sans wouldn’t be that ignorant.

Would he?

“Are there any questions?”

Papyrus let out a breath in relief. Everyone shook their heads, congratulating the worker on finishing up their bosses presentation so smoothly.

Assuming he was no longer needed, Papyrus slipped out the door.

He didn’t feel the set of eyes watching him as he left.

*~*~*~*~*~*~*~*~*~*

“Boss?” Papyrus knocked on the door. “Hey, boss? It’s Papyrus.”

No answer.

“Boss?”

Still nothing.

“Sans?”

Papyrus opened the door. His breath hitched when he looked inside and he quickly stepped in and shut the door.

Sans was still lying on the floor, his body shaking. Papyrus moved quickly to him and went to roll him over but he hissed as soon as his hands made contact with the smaller body.

“You’re on fire,” Papyrus murmured. “Sans, what’s going on?”

“N-n-nothing,” Sans stammered. “I’m fine, Papy, just, it’s too warm.”

Papyrus sighed with a touch of exasperation. “Boss, you’re still in heat.”

“No, I’m not,” Sans said, pushing to sit up with a wince. “Took care of that with that jerk.”

Papyrus shook his head. “No, that must have just made it fade for a bit. You didn’t satisfy it.”

“I’m fine, Papyrus, I swear,” Sans insisted, waving a hand dismissively.

“You’re not!”

Sans scowled. “I can handle this myself!”

“Is that what you’ve been doing? Because it looks to me you’ve been trying like hell to repress it and you are _failing_.”

Sans pushed up to his feet, shakily. “You don’t need to worry about me. I got it covered, it should go away soon.”

Papyrus stared at him. He slowly got to his feet, still keeping eye contact. “Sans, whatever you are doing to ‘take care of this’ isn’t working.”

Sans scowled again. “How would you know?”

Papyrus was getting frustrated. “I checked you, Sans. You’re going to hurt yourself! Possibly worse!” Papyrus huffed. “Actually, there is no possibly, you’re going to get yourself killed!”

Sans tried to muster up anger, but he couldn’t. All he could feel was burning. He finally clenched his fists, determined to control it, though when he looked into Papyrus’ face, another feeling washed over him and he had to bite back a groan.

“Y-you need to leave,” Sans stammered.

Papyrus put his hands on Sans’ shoulders, but Sans flinched away outside his reach.

“Don’t touch me!” Sans moved to his door and reached for the handle. “You need to go, Papyrus. Don’t worry about me.”

Sans gasped in shocked as an arm blocked him from reaching the doorknob. The smaller skeleton looked up and saw Papyrus towering over him with a strange look on his face.

“Sans, seriously, you are going to die.”

Sans’ whole body shook. He knew Papyrus was right; if his heat wasn’t satisfied, the heat would build in his soul until it literally burst. The outpouring of magic tended to be dangerous and generally caused a good deal of harm to the owner of the soul, but Sans was a special case. He only had one HP, and a heat explosion would surely be his end.

But his pride was in the way.

“I can do this myself. Now get out of here before I-!”

Papyrus pressed his teeth against Sans’ while the hand he used to block him from the doorknob reached over and locked the door. Sans broke the kiss in a slight panic, glancing over at the doorknob, but that only left his neck open for Papyrus to lick at, drawing a loud moan from the smaller skeleton.

After a moment, Sans made to move away, so Papyrus reached up with his other hand and pressed it to Sans’ chest, pressing him against the door.

“I’m sorry, Sans, I can’t risk losing you,” Papyrus murmured against his skull.

Sans moaned under him and shook his head. “No, Papyrus, I don’t need you to…”

“Yes, you do,” Papyrus started undoing Sans’ pants. “Damn it, Sans, I’m not losing you, not when I can do something. I, I’m sorry.”

Sans’ protest died in his throat as Papyrus yanked his pants down and ran his tongue along one edge of his pelvis. Sans’ head hit the door hard as he cried out, hands flailing before clawing at the thick wood behind him.

Papyrus was a little shocked at how hot Sans’ bones were under his hands and tongue and wondered how long he had before Sans was at the point of no return. He channeled his concerns into his actions; getting Sans to a safer level was going to take more than a blowjob, and maybe more than a round or two of full-on sex, but for now it didn’t matter. Papyrus knew he had to get Sans off at least once as quick as possible.

Not like Papyrus was too upset about it.

Sans continued to moan and squirm against the door as Papyrus kept touching and licking around his pelvis. It took longer than Papyrus expected for Sans’ magic to respond and give him something else to work with. He figured (hoped) it was just Sans being stubborn.

Papyrus wasted no time bringing the cyan-colored cock into his mouth, swirling his tongue around it, and not expecting to really like the whining cries that escaped from Sans with every weak breath.

“Pa-Pa-Papy,” Sans somehow found the ability to cry, his voice sounding horse and quiet. “Pl-please…”

Sans’ hands weakly touched his shoulders, but there was barely enough strength in him to make contact much less push him away.

Papyrus could still feel Sans resisting; damn was he ever stubborn. He replaced his mouth with his hands, pumping Sans slowly as he moved up to look into his eyes.

“Sans, please, relax and let me do this,” Papyrus said. “I’m not going to hurt you.”

Sans’ eyes were only half-open as he dragged his pupils up to look at him, blue and yellow magic was swirling chaotically in their depths. There was still a slowly dulling spark – Sans’ resistance.

“Please? I can’t let you die, but I really don’t want to… I prefer your permission.”

Papyrus leaned in to nuzzle the side of Sans’ face, his hand was still slowly moving between Sans’ legs, and he was hoping that his boss would come to his senses for just long enough to give consent.

Sans let out another low moan, pressing his face against Papyrus’ as a few tears fell.

“Papyrus,” he forced out, voice barely above a whisper. “Help me, please.”

The tight band of worry and fear that had Papyrus in a vice-like grip since he had “checked” Sans began to loosen as he let out a long sigh of relief. He reached up with his free hand to cup Sans’ cheek, his thumb wiping away the tears.

“Thank you for trusting me,” Papyrus murmured.

Sans’ eyes descended into pure chaos and he leaned in to kiss Papyrus as his hips jerked forward into his grasp. Papyrus obliged and moved his hand faster as he continued to slowly ravage Sans’ mouth.

It didn’t take long, in fact Papyrus was a little surprised when Sans let out a small cry, breaking their kiss and warmth flowed over his hand.

“That’s it, let it out,” Papyrus murmured, continuing to pump the twitching cock in his hand until it stopped leaking.

Sans leaned his head back against the door, breathing hard but quietly.

Papyrus leaned in and licked his neck; he noticed Sans had cooled a bit, but was still incredibly warm.

“You’re far from done,” he murmured.

Sans’ eyes squeezed shut; he was still trying to thrust into Papyrus’ hand, though weakly and with stuttering movements.

Papyrus ran his hands down Sans’ sides, one side leaving a streak of blue-tinged magic. Now that Sans had cooled a little, Papyrus wanted to take a moment to simply touch the body he knew he would have to screw into oblivion (not an unpleasant notion by any means). He felt the thick rib bones beneath the crisp shirt and soft undershirt. He traced back up to press along his clavicles and gripped his shoulders. Sans may be small, but he was solid.

Sans moaned and moved into the touches and Papyrus could feel his heat building again. He pushed down his own pants, feeling his own magic starting to form and he pressed his body against Sans’, pinning him to the door. San squirmed against him, panting, his hands clawing at Papyrus’ chest.

“Please, please Papy,” Sans begged.

“I got you,” Papyrus murmured, one hand reaching down to rub between Sans’ legs as he leaned down to kiss him again, his other hand pressing against his shoulder to keep him up against the door.

Papyrus reached a little further back, finding a small entrance conjured and slowly pressing his finger inside. Sans let out another low cry, his eyes squeezing shut.

“Are you okay?” Papyrus asked.

“Stars, yes,” Sans whispered shakily.

Sans’ magic made the tight tunnel slick and pliant, there wasn’t much resistance when Papyrus gently pressed two fingers inside.

‘ _Wow, this is really bad for him,_ ’ Papyrus thought. Sans tried to press against him, as if he could hear his thoughts and wanted to emphasize how correct he was.

Papyrus reached under Sans’ pelvis with both hands and picked him up. Sans wrapped his legs around his hips as he was pressed harder against the door. Papyrus steadied him with one hand while reaching down with the other. He stroked himself a few times before looking into Sans’ eyes. Nothing had really changed.

“All right, you ready?” Papyrus asked.

Sans answered with a moan that was laced with pain. “Make it stop, please.”

Papyrus nuzzled his face. “All right. I will. I’m going to make you feel so much better.”

He didn’t bother waiting but he tried to be as gentle as he could as he steadily pushed inside the willing body.

Sans’ breathing picked up and tears poured from his eyes again as Papyrus pressed deeper inside him. He babbled in incoherent stutters, his legs squeezing around Papyrus’ hips, trying to push him in deeper and faster while his hands clawed at Papyrus’ back.

Papyrus let out a cry of his own; yes, Sans was wet, nearly dripping, but he was so tight and so hot; the heat was overwhelming. Papyrus pressed his head into the door over Sans’ shoulders when he was all the way inside, trying to breathe.

“Sans, stop squirming,” he muttered after a couple of minutes when Sans grew impatient with his stillness.

Sans opened and closed his mouth, trying to speak, but only squeaks and gasps came out and he still squirmed.

Papyrus growled in annoyance and leaned down to bite Sans’ neck, making him cry out in pain, but it made him go still.

“Sorry,” Papyrus breathed the apology after a few moments. “You’re just so hot and tight, but…”

Papyrus finally moved and the both of them groaned. Papyrus moved again and Sans’ head leaned back on the door, his mouth hanging open a little. Papyrus leaned in to lick at the bite mark he accidentally left, but somehow he felt a little proud of it - he marked what was his.

If he wasn’t drowning in the sensations of his heat and Papyrus moving in and out of him, Sans would have worried about them banging their bones against the door, but he didn’t care. Papyrus was thick and heavy inside of him, rubbing and pressing against all the right spots and Sans never remembered feeling such completeness, such relief, such an overwhelming feeling of everything being right.

Papyrus could feel himself approaching his edge and he reached for Sans’ hands, bringing them up one at a time to pin above his head.

“You ready to cum?” Papyrus growled low near his ear.

Sans nodded with a whine.

Papyrus smirked and reached down with his other hand to squeeze Sans’ cock.

“Don’t hold back on my account, we still have a long way to go,” he licked up the side of his neck. “Cum for me Sans and let me hear it.”

Sans whined louder, his head pitching forward for a moment before he suddenly threw it back against the door.

“FUCK! PAPYRUS!” he all but screamed.

Papyrus was almost forced to still as Sans tightened around him, trying to burying himself as deep inside him as he could. Warmth spilled over his hand again, a bit even sliding down his leg; Papyrus would have never guessed the little guy had it in him.

With Sans’ inner walls massaging him, his own release was triggered and he came with a loud moan. He let go of Sans’ hands, which immediately wrapped around his shoulders, his legs still squeezing him, and Sans pressed his face into Papyrus’ chest. They both shook with the aftershocks of orgasm, Papyrus only keeping them upright by leaning them both against the door.

“P-Papyrus,” Sans stammered after a couple of minutes. “A-are you okay?”

Papyrus took in a deep breath and let it out slowly. “Oh, yeah, absolutely,” he pressed his forehead against the door, looking down at Sans. “You’re getting there; you sound a little more coherent.”

Between the embarrassment and shame beginning to swell in him, Sans could only nod, unable to look up.

Sensing the worrying emotions, Papyrus pushed off the door with one hand while wrapping under Sans with the other. He stepped carefully out of his pants and moved them to the couch. Their genitalia had faded at this point, though some stickiness remained between them. Papyrus sat down carefully with Sans in his lap, and Sans tried to move to sit next to him but Papyrus’ grip was firm.

“Stop it,” Papyrus said firmly. Sans glanced up with a questioning gaze. “I know what you’re thinking. I can feel the guilt coming off you as much as I can still feel your heat. You have done nothing shameful, unless you count your stupid pride nearly getting you killed.”

Sans stared, letting the words sink in, but when they did, he leveled a glare at his assistant.

“You checked me,” Sans said in a low, angry tone. “How could you? That’s such a blatant invasion of privacy! I didn’t even know you could do that!”

Papyrus sighed, sinking back into the couch and loosening his grip on Sans, letting his hands rest loose on his hips. He knew this would come up eventually.

“Yeah, all right, for what it’s worth, I am sorry,” Papyrus said. “It’s something I taught myself how to do back when I was a kid. I don’t make a habit out of doing it, it does take a bit of energy, and you know me,” he gave Sans a wide grin, who returned it with a very small smile. Papyrus reached up to caress the side of Sans’ head. “I just got concerned today, and I would have somehow cornered you and talked instead of checking, but there was no time. Seriously, boss, you were killing yourself.”

Sans couldn’t meet Papyrus’ eyes. “I, I hate worrying people.”

“Well, I’m sorry but you worried people anyway. Or me, at the vey least,” he gave Sans an odd look. “So, I guess fucking McEdgeLord didn’t do much?”

Sans’ face immediately flushed a pale blue. “I’m not talking about it.”

Papyrus chuckled. His eyes were drawn to a spot on Sans’ neck that was beginning to turn colors. He winced and gently ran a finger over it.

“Erm, sorry about that, too,” he murmured. “I hope the collars of your shirts will cover it.”

Sans reached up and traced his own fingers over the bite mark, feeling a little pain, but it triggered another flush as the thought of being marked screamed in his mind.

Along with the flush came another wave of heat and Papyrus was reminded why they were sitting on the couch half naked.

With little warning, he turned and pushed Sans onto his back on the couch.

“I know you’re feeling better, but I know you’re not finished,” he leaned down and kissed him gently on the mouth. “Shall we?”

Sans knew better than to protest as the haze of his heat soon flooded his mind once more.

 

**Author's Note:**

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